QX1 solves the multigroup, one- dimensional, time-dependent diffusion equations. Problem geometry may be plane, cylindrical, or spherical. Steady-state initial conditions may be established either for a source-free system or for a system with an external neutron source. The reactor may be perturbed by changing material volume fractions and/or temperatures or by changing the neutron source level. A first-collision pulsed source distribution may be specified. Resonance absorption feedback is calculated by groupwise interpolation in a cross-section versus temperature table. A highly simplified fuel temperature model is included.

The improved quasistatic method described in reference 1 is used to solve the time-dependent problem. The method consists of factoring the total flux into the product of a space- energy-time dependent shape function and a purely time-dependent amplitude function, normalized so that the most rapidly varying part of the total flux is included in the amplitude function. The two coupled sets of equations which result are solved iteratively. The fuel temperature changes are calculated by a regionwise adiabatic model with the assumption that all power is produced in the fuel. The method was developed specifically for fast reactor safety analysis. The advantages of factorization are greatest for such systems, though the code has been shown to perform adequately on thermal reactor problems.

A 29-group, 15 downscatter, 51 mesh point rod-drop problem run to 30 reactor seconds executes in 10 minutes on the IBM360/75. A 10-group, 6 downscatter, 53 point pulsed reactor problem run to 1 msec. reactor time executes in 10 minutes on the IBM360/75. Running time estimates cannot be generalized because of the high degree of dependence on the required accuracy of solution, type of perturbation, and the eigenvalue separation of the space-energy equation.

The running time can be reduced greatly for problems requiring relatively low accuracy, and the code has been shown to reproduce the results of direct finite-difference codes when convergence is tightened. An automatic time-step selector is provided to optimize the time distribution of shape function recalculations during the transient. A true point kinetics problem can be run using only the initial shape function. A compact problem edit is given in terms of the familiar integral quantities of reactivity, effective delayed-neutron fraction, generation time, etc. Very general problem driving functions and time-step controls may by used. A group-collapsing system is built into the problem preparation module of the code.